Choosing a protector

[SystemeD]

Personally I think that the only thing that could not be cracked is the thing that isn't on the reverser hd.
So if possible in your case, you can try to release a demo version compiled without all the features and components of the full version and send the complete application only after payment (protected in some way, with info of the customer in it and linked to the demo version installed by the customer). Then you have to hope that all your customer are satisfied and honest and would not spread their full version...
Anyway it's true that there always will be persons that would use your app without paying for it so try to make a great app that could gain the respect of your customer.
Regards